The present invention relates to an improved cup. More particularly, it is concerned with a cup which is of high whiteness, has a high quality paper-like appearance of reduced luster and transparency impression, is high in strength, stiffness, heat resistance, and stability, and furthermore which is greatly improved in disposability and ease of disposal by burning.
Disposable cup such as food cups and drink cups, particularly those for use in a vending machine and a picnic have heretofore been made of paper, synthetic resins, or laminated paper.
Cups made of paper are light-weight and are easy to dispose by burning but have disadvantages in that water resistance is inferior, productivity is low because they require a bonding step in the production thereof, and furthermore in that the choice of a suitable adhesive is limited by its toxicity; that is, an adhesive to be used should be selected taking into consideration safety for handling or hygienic packaging.
For this reason, laminated paper prepared by coating or laminating synthetic resin films or paper are used. Cups made of these laminated paper are improved in water resistance to some extent but cannot be said to have been substantially improved because they are not made in one body; that is, they have bonded sections. Since the process of production of these cups are substantially the same as for production of paper cups except that paper is replaced by laminated paper, productivity is low, leading to an increase in production cost.
Cups made of paper or laminated paper can be produced only in relatively simple forms such as in a form circular or square in cross section because of the characteristics of paper. Moreover, since they cannot be molded in one body, it is inevitably necessary to bond together parts in the fabrication thereof. When they are used in a vending machine, they do not always drop in completely the same manner because of irregularities due to bonded areas and of light-weight. That is, irregular dropping is liable to occur, causing problems and, therefore, severe quality control is needed in the fabrication of the cups. In the case of printed cups, a problem arises in that the print is deviated in bonded areas.
In recent years, therefore, cups made of synthetic resin sheets by thermoforming have begun to be used. These cups are not sufficiently satisfactory in heat resistance and stiffness although they are superior in thermoformability, light-weight, sanitariness, and so forth. In connection with the appearance, they have luster characteristic of synthetic resins, and they are inferior in opacifying properties, printing properties, and so forth. Thus, there has not been obtained a cup which gives a high quality impression as in paper cups. These cups made of synthetic resin sheets cannot be collapsed and become very bulky in the disposal thereof. Moreover, the calorific value of such cups made of synthetic resin sheets is much greater than that of paper. Thus, in disposal by burning, they damage the walls of a incinerator and cause melting and solidification. Depending on the type of resins used, they produce poisonous gases and black smoke, causing serious social problems in their disposal.
In order to overcome the above-described problems, an attempt has been made to produce cups by using a procedure which involves compounding an inorganic filler to a polyolefin resin, melt-kneading the resulting mixture, molding it into a sheet by the calendering process or extrusion molding process, and subjecting the sheet to fabrication such as vacuum molding and/or pressure molding. These cups are relatively high in stiffness and heat resistance and also are superior from a viewpoint of prevention of pollution in that they have low calorific values, do not cause melting and solidification, and in that they do not produce black smoke. Furthermore, addition of such inorganic fillers reduces the amount of synthetic resins used. Thus, these cups are expected to be in widespread use.
Cups produced by thermoforming, however, are relatively low in whitness and, because of their characteristic luster, inevitably give a strong impression that they are made of synthetic resins. Particularly, in the case of thin wall cups measuring 1 mm or less in thickness, the transparency impression remains and the appearance is not sufficiently satisfactory. That is to say, such cups do not give a high quality impression as in paper cups. In the case of cups produced using calcium carbonate, which is the most commonly used inorganic filler, the problem that calcium carbonate elute from the cups arises. Thus these cups are not suitable for use as food cups. In practice, therefore, cups made of inorganic filler-compounded polyolefins are not used at all in the field of drink cups although they have superior characteristics and high productivity.
In producing cups by thermoforming, such as vacuum forming, of sheets of thermoplastic resin compositions in which an inorganic filler is compounded in a relatively large amount, the formation of irregularities in the surface of the sheets due to extrusion at the time of production of sheets occurs and deep draw forming is difficult. In particular, when thermoforming is performed at relatively low temperatures, the inorganic filler contained elute to the surface of the cup, reducing surface characteristics. When the cups surface is printed, particularly when a cup which is circular in cross section is subjected to high-speed and continuous printing by the use of a curved surface printing machine, the inorganic filler solving out of the cup is transferred to the blanket part coming into contact with the cup. With a lapse of time, the thus-transferred inorganic filler moves to rollers and ink, making it impossible to produce beautiful prints stably over long periods of time. Depending on the purpose for which the cup is used, the inorganic filler and other additives are dissolved in the contents such as drinks. In the case of a cup for soft drinks, carbon dioxide gas is lost with a lapse of time, resulting in a reduction in flavor.